In a typical wireless communications network such as a cellular radio system, wireless terminals (also referred to as user equipment unit nodes, UEs, and/or mobile stations) communicate via a radio access network (RAN) with one or more core networks. The RAN covers a geographical area which is divided into cell areas, with each cell area being served by a radio base station (also referred to as a RAN node, a “NodeB”, and/or enhanced NodeB “eNodeB”). A cell area is a geographical area where radio coverage is provided by the base station equipment at a base station site. The base stations communicate through radio communication channels with UEs within range of the base stations.
In such a communications network, a data rate that can be supported over a target link TL between a base station and a wireless terminal may be limited by interference from other sources. As shown in FIG. 1A, for example, base station 19a may receive uplink communications from wireless terminals 11a and 15a. In receiving the uplink communications from wireless terminal 11a (referred to as a target link TL) at a receiver of base station 19a, uplink communications from another wireless terminal 15a (referred to as an interfering link IL) may interfere with reception of the uplink communications from wireless terminal 11a. As shown in FIG. 1B, base station 19b-1 may transmit downlink communications to wireless terminal 11b, and base station 19b-2 may transmit downlink communications to wireless terminal 15b. In receiving downlink communications from base station 19-1b at a receiver of wireless terminal 11b (referred to as a target link TL), downlink communications from base station 19b-2 to wireless terminal 15b (referred to as an interfering link IL) may interfere with reception of the downlink communications from base station 119b-1 to wireless terminal 11b (the dashed line in FIG. 1B indicates the interfering link as perceived/received at wireless terminal 11b). As shown in FIG. 1C, base station 19c may transmit downlink communications to wireless terminals 11c and 15c. In receiving downlink communications from base station 19c at a receiver of wireless terminal 11c (referred to as a target link TL), downlink communications from base station 19c to wireless terminal 15c as (referred to as an interfering link IL) may interfere with reception of the downlink communications from base station 119c to wireless terminal 11c (the dashed line in FIG. 1C indicates the interfering link as perceived/received at wireless terminal 11c).
In any of the examples of FIGS. 1A, 1B, and/or 1C, a received signal at a target link TL receiver (at a wireless terminal or at a base station) may include a target link TL with information intended for the target link TL receiver and one or more interfering link IL or links. A ratio of received signal power of the target link TL to a received signal power of an interfering link IL or links (as received at the target link TL receiver attempting to receive the target link) plus other noise and interference, may be referred to as a geometry factor. The geometry factor may be a significant factor determining an achievable data rate for the target link. Stated in other words, a greater geometry factor (i.e., a greater ratio of target link TL signal strength to interfering link IL signal strength at the target link TL receiver) may allow a greater data rate to be transmitted over the target link TL to the target link TL receiver than a lower geometry factor (i.e., a lower ratio of target link TL signal strength to interfering link IL signal strength at the target link TL receiver). By reducing an effective power of an interfering link IL at a receiver (which may result from traffic data transmissions), an effective geometry factor for the target link TL at the receiver may be increased/improved, thereby improving receiver performance and/or allowing increased data rates. An effective power of an interfering link IL, for example, may be reduced using linear suppression, pre-decoder interference cancellation, or post-decoder interference cancellation.
With linear suppression, the target link TL receiver may include multiple receiver (RX) antennas, and an antenna lobe diagram may be steered so as to point a spatial null in an arrival direction of a dominant source of interference. Statistics of the received signal may be used to determine combining weights leading to the desired spatial pattern, e.g., using Interference Rejection Combining (IRC) to provide improved Signal-to-Interference-and-Noise Ratio (SINR).
To significantly suppress the interfering link IL with a 2-antenna receiver, the interfering link IL should arrive from a well defined single direction. In dispersive environments where several reflections from different directions may contribute, however, null steering may not be effective. Moreover, if the null steering degree of freedom is used to suppress the interfering link IL, this degree of freedom may no longer be available for spatial InterSymbol Interference ISI suppression or inter-stream interference suppression in multiple-input, multiple-output (MIMO) transmissions on the target link TL, thus significantly lowering the equalization efficiency.
With pre-decoder interference cancellation, the receiver may demodulate the interfering link IL from a received signal and apply hard decisions to the symbol estimates resulting from the demodulation to reconstruct the transmitted symbol sequence. The reconstructed symbol sequence for the interfering link IL may be filtered with the channel and subtracted from the received signal. After that, the desired target link TL signal may be demodulated and decoded from the received signal. By providing interference cancellation, the target link TL signal may be demodulated and decoded with higher quality than without interference cancellation.
Pre-decoder interference cancellation may be effective when the raw symbol SINR of the interfering link IL at the target receiver is sufficiently high to make reliable hard decisions. If the raw symbols of the interfering link IL are not sufficiently reliable, however, applying hard decisions may lead to significant decision errors and to interference amplification instead of cancellation.
With post-decoder interference cancellation, the receiver may demodulate and decode the interfering link IL from a received signal. The resulting decoded bit sequence for the interfering link IL may then be re-encoded, and the coded bits may be passed through a modulator to reconstruct the transmitted symbol sequence for the interfering link IL. The reconstructed sequence may then be filtered with the channel and subtracted from the received signal. After that, the desired target link TL signal may be demodulated and decoded from the received signal. By providing interference cancellation, the target link TL signal may be demodulated and decoded with higher quality than without interference cancellation.
Post-decoder interference cancellation may be effective when the Modulation and Coding Scheme MCS applied to the interfering link IL is sufficiently conservative (with e.g., sufficiently low code rate) for the TL receiver to be able to demodulate and decode. If radio conditions of the interfering link IL between the interfering link IL transmitter and the intended interfering link IL receiver are better than radio conditions of the interfering link IL between the interfering link IL transmitter and the target link TL receiver, the target link TL receiver may not be able to successfully decode the interfering link IL transport block. This situation may be detected using error detection/correction (such as a Cyclic Redundancy Check or CRC) and any degradation due to incorrect IC feedback may be avoided, but no target link TL geometry improvement will be achieved.
Each of the interference cancellation techniques discussed above may thus be unable to provide a high level of interference cancellation over a full range of varying radio conditions.